A good example of a C++ feature that depended on runtime support and didn't get timely support on older OSes was thread_local. Apple OSes didn't support it for a good few years after upstream clang and libc++ added support, in Xcode 8, and you still needed to target iOS 9 or OS X El Capitan to use it.
On the Swift front, we are continuing to work on functionality that will make it easier for platform vendors to back-deploy new APIs:
It would be interesting to see this back deployment feature extend to other declarations such as new types in the future too.